Large internal combustion engines that are used for instance in ships and power plants are usually started by using pressurized air that is injected sequentially into the cylinders of the engine for rotating the crankshaft. It is desirable to minimize the amount of starting air that is needed for starting an engine. Pressurized starting air needs to be stored in large air receivers that require a lot of space. The energy needed for pressurizing the starting air is usually taken from the engine itself, and it is therefore also desirable to minimize the energy consumption for this purpose.
To minimize the consumption of starting air, it is important to select the most suitable cylinder for the initial starting air injection. To be able to do this, the crank angle of the engine must be determined. Engines are often equipped with crank angle sensors that can be used for instance for determining correct fuel injection timing. These sensors are usually incremental encoders, which determine the crank angle on the basis of detection of a reference mark and angular changes of the crankshaft. This means that the crank angle cannot be determined when the engine is at rest. One method for determining the crank angle is to rotate the engine before start so that a reference mark passes the crank angle sensor and the absolute crank angle is thus known. Then the engine has to be rotated further into a starting position. A drawback of this method is that the method is time consuming and a lot of energy is needed. Due to vibrations, the method can also be inaccurate.
An alternative way is to use an absolute rotary encoder to determine the crank angle of the engine. With an absolute rotary encoder, the exact crank angle of the engine can be determined even when the engine is at rest. Patent application US 2007005222 A1 discloses an air start-up system for an internal combustion engine. The system comprises solenoid-controlled starting valves in connection with each cylinder of the engine for introducing starting air into the cylinders. An absolute rotary encoder is used to detect the angular position and rotational speed of the engine for determining the correct timing and duration of starting air injection.
A problem with absolute rotary encoders is that they are fragile and not intended for extended periods of rotation at high speed. Therefore, the lifetime of an absolute rotary encoder is often very limited.